1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask, a reticle, an array of individually controllable elements, etc. can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer or a flat panel display substrate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the liquid can also be regarded as allowing the numerical aperture (NA) of the system to be higher than 1 and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g., quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid can lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate (the substrate generally has a larger surface area than the final element of the projection system).
A gap between the liquid supply system and the substrate allows these elements to move with respect to each other. Because of this gap, there is a need to have high surface tension between the immersion liquid and at least a “showerhead” portion of the liquid supply system to keep the immersion liquid from flowing through this gap. For example, a showerhead can be a portion of the liquid supply system that comprises inlet and outlet ports and/or channels. A problem that can arise in immersion lithography systems small contact angles between an immersion liquid and a surface of the substrate and the liquid supply system. This can be due to wetness effects of using certain materials for the liquid supply system and/or the immersion liquid. Small contact angles can lead to low surface tension between the immersion liquid and the showerhead. Thus, the immersion liquid may flow over the entire substrate surface or leak through the gap, which are both highly undesirable.
A still further problem is that certain immersion liquids cause corrosion of the liquid supply system when it is made from steel. Corrosion can contaminate the immersion liquid and cause defects on the substrate surface. One way to alleviate this problem is by changing of the immersion liquid. However, changing the immersion liquid can lead to other problems, such as wetness problems.
Therefore, what is needed is a system and method that allow for reduced wetness characteristics without causing contamination of the immersion liquids.